1. Field of the Invention
This invention relates to transducers used in seismic exploration and more particularly to springs used in such transducers for suspending an inertial mass within a magnetic field.
2. Discussion of the Related Art
Geophones typically used in seismic exploration include a spring for suspending an inertial mass in a supporting frame, and a means for measuring the relative motion between the inertial mass and the frame. Typically the inertial mass is a wire-wound coil form suspended by springs in a magnetic field, one spring being attached to each end of the coil. The springs centrally position the coil form within the magnetic field both laterally and vertically along the geophone's axis. The springs and mass of the coil affect the resonant or natural frequency of the system.
Several conventional geophones use springs having an outer ring and an inner ring interconnected by a plurality of legs. Commonly, many such springs are formed by etching or stamping the spring from sheets of spring material such as beryllium copper. Historically, a wide variety of geophone springs have been consider implemented, ranging from those having as many as nine legs to those having as few as three legs. Other springs utilize a cantilevered construction having concentric rings or adjacent arcuate legs similar to modern geophones. Still others employ a combined arcuate- and straight-segmented leg construction. Each of the above springs are disclosed in U.S. Pat. No. 2,348,225 (six arcuate legs); 2,751,573 (straight leg); 3,020,767 (straight leg); 3,602,490 (concentric cantilevered legs); 3,738,445 (adjacent arcuate and cantilevered legs); 3,890,606 (overlapping arcuate legs); 4,152,692 (concentric arcuate legs); 4,238,845 (compound arcuate legs); 4,323,994 (straight-segmented legs); 4,458,344 (tapered, straight-segmented legs); and 4,623,991 (delta shaped spring).
The legs of geophone springs generally have a rectangular cross-section and are curved or bent at specific locations in order to join the inner and outer portions of the spring. After etching or stamping, the spring is preformed according to known techniques to offset the inner portion or annulus from the plane of the outer annulus. When the inertial mass is suspended between two such springs, the inner ring, legs, and outer ring of each spring lie in the same plane.
A geophone is intended to detect motion in a direction roughly parallel to the axis of the coil form within the geophone housing. Therefore, the effects of any lateral motion of the coil form in response to forces perpendicular to the axis of the suspended coil are undesirable and should be eliminated or minimized. Such is a common occurrence in the handling of the geophones during a seismic survey.
Seismic signals which arrive substantially parallel to the axis of the geophone cause the geophone to generate a primary signal or response. Seismic signals which arrive substantially perpendicular (cross-axial) to the axis of the geophone cause the geophone to generate a spurious response. The spurious resonance is normally defined as the first major amplitude peak on the frequency spectrum of the spurious response which is broadband. It is desired that the spurious resonance be as high as possible so as not to be confused with signals arriving substantially parallel to the axis of the geophone.
There are two contributing factors to the spurious response, with relative levels dependent on the individual design of a particular model of geophone. One component of the spurious response is due to a complex transformation of forces within the spring which result in an axial movement of the coil, causing the coil to pass through radially directed lines of magnetic flux. The other component of spurious response is due to a cross-axial movement of the coil through the lines of magnetic flux which are not purely radially directed.
In modern geophysical prospecting, frequencies greater than 100 hertz (Hz) are of increasing interest to the geoscientist. However, conventional geophones may not be used to detect signals at such frequencies because of inherently low spurious resonances. It has been found that the spurious resonance of the geophone can be raised and lowered by changing the geometry of the geophone springs. One method for raising the spurious resonance of the geophone is to decrease the length of the legs connecting the inner and outer annuli. Unfortunately, this method can decrease the linearity of the spring to the extent its response becomes distorted. Unfortunately, the signal distortion, caused by a spring's nonlinear behavior, is increased when the legs are relatively short. Attempts to achieve the optimum leg ratio to provide high spurious resonance and yet maintain lateral stiffness are disclosed in U.S. Pat. Nos. 4,323,994, 4,458,344, and 4,623,991.
Accordingly, it is an object of the present invention to provide a geophone spring having a spurious resonance above the frequency of interest in seismic surveys. It is another object of the present invention to provide a geophone spring having improved lateral stiffness while providing the desired linear behavior and increased spring life.